Almost all governments, in all countries, over the last hundred years have done so.
sales tax
13.55
Mark-up, it is not profit. Profit must account for other fixed costs associated with selling
The two added together are the cost of the item to you.
$99.87
16
The item will cost 170.00
In auction, it is a percentage added to the cost of the item sold under the hammer. It is the auctioneers expenses charge
Multiply the cost by 0.75 and that is the sale cost of the item.
$21.60
50
50%